T Wooden Structures 1

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1 T Wooden Structures 1 Load Bearing Wooden Structures Structure of Wood Structural Properties of Wood Timber Glued Laminated Timber Laminated Veneer Lumber 1

2 T Wooden Structures 1 Load Bearing Wooden Structures Structure of Wood Structural Properties of Wood Annual growth -> Annual ring Early wood, latewood, heartwood Orthotropic material: Orthogonal values of elasticity and strength properties are different Natural anisotropy: - longitudinal direction of grains - tangent of annual ring - perpendicular of annual ring Structural anisotropy: - e.g. veneers of plywood (thickness, direction of grains, quality, position) 2

3 Theory of Elasticity - Hooke s law (σ = E ε ) - the effect of transformations in stress distribution is considered only in stability check - material will break, when the stress is equal with the strength - stress distribution has no effect on the strength of material - designing is simple Reality - Hooke s law is valid when stresses are relatively small - Young s modulus differ in tension and in compress - material will become plastic before breaking 3

4 Loaded wood behaves as viscoelastic material Deformations of loaded wood increase in time, wood creeps! The strength will decrease and the deformations will increase with increasing moisture (absolut humidity of wood 0 30%) in analysis the strength and stiffness values are adjusted according to the relative humidity of conditions and duration of loads with factors k mod and k def 4

5 Shrinkage and swelling The changes in humidity also cause deformations, ortotrophy! Direction Temperature [1/ C] Factor Humidity (change / 1%, when 0%... 30%) Grain 5*10-6 1*10-4 Tangent of annual ring Perpendicular of annual ring 45* * * *10-4 Example: shrinkage of pine (30% 0%) 8% 0,2 % 4% 5

6 Strength grading of timber The strength of timber vary Visual grading: - number, location and quality of knots, the most important parameter (~90%) - cracking, twisting, skewness - density, proportion of latewood Mechanical grading: - bending test (+visual) - Young modulus (of elasticity) 6

7 Strength grading of timber Test conditions in eurocode: 300s, T=20 C, RH65% Strength varies distribution of values: - the value for analysis in ultimate limit state is the lower 5% fractile of strength distribution - example graph: distribution of tension strength in grades (a), (b) ja (c) 7

8 Strength grading of timber Visual grading of solid timber courses for certification (personal certificate) Mechanical Grading certification Finger jointed timber certification The certificates are granted by officially approved institutions e.g. Inspecta Sertifiointi Oy ( or VTT Technical Research Centre of Finland ( 8

9 Quality grading of timber Don t mix with strength grading! Quality grading is based on exterior features of timber: - knots, cracks, wanies, resin and bark pockets or wounds, angle grains, grown breaks, compression wood, soft rot and wrong shapes I - VI (first sixth grade) or A(A1-A4) -D (scandinavian) 9

10 Strength of timber Strength grades (pine, spruce): - EC grades: C14,C16,C18,C20,C22,C24, C27,C30,C35,C40,C45,C50 - Finnish grades (old): visual T18,T24,T30,T40 mechanical MT18,,MT40 - Scandinavian INSTA 142 T-grading: T1,T2,T3 (= C18,C24,C30) 10

11 Strength of timber Descriptive example: (not to be used in real analysis!) 11

12 Strength of timber Service strength and stiffness values are tested according to eurocode. Test conditions: - relative humidity 65% RH - temperature 20 C - duration 300s (5min) Characteristic values of strength are adjusted to the size of cross section with the factor k h, if the height of the beam in bending or longer side in tension (h) < 150 mm. 12

13 Timber Service strength [N/mm 2 ], example chart (RIL 205): Strength grade C14 C24 C30 C40 Bending f m,k Tension f t,0,k Tension f t,90,k Compression f c,0,k 8 0, , , ,6 26 Compression f c,90,k 2,0 2,5 2,7 2,9 Shear f v,k 1,7 2,5 3,0 3,8 f v,90,k = 0,5 f v,k Factor k h, if the height of the beam in bending or longer side in tension (h) < 150 mm k h = min ( 150 / h) 0, 2 1,3 13

14 Timber Service stiffness [N/mm 2 ] and density, example chart (RIL 205): Strength grade C14 C24 C30 C40 Modulus of elasticity: E 0,mean E 0,05 E 90,mean Modulus of shear: G mean Density [kg/m 3 ]: ρ k ρ mean NB! Old, visual strength grading: : T18 ~ C18 T24 ~ C24 T30 ~ C30 14

15 Glued Laminated Timber According to EC386 the laminated timber sheets are usually 45 or 33mm thick, at least 4 sheets! The timber has to be strength graded The glues (resorcinol-pfenol, melamine) are weather proof (except polyurethane) 15

16 Glued Laminated Timber Scandinavian strength grading: L30, L40 EC: GL28c,GL28h,GL32c,GL32h, h = homogeneous timber c = combination of different strength grades in cross section, see below Example L40 (~ EC: GL32c) 16

17 Glued Laminated Timber Service strength and stiffness values are tested according to eurocode or analysed according to the glued sheets. Test conditions (EC): - relative humidity 65% RH - temperature 20 C - duration 300s (5min) Characteristic values of strength are adjusted to the size of cross section with the factor k h, if the height of the beam in bending or longer side in tension (h) < 600 mm, and in tension (perpendicular to grain direction) if the volume is > 0,01 m 3 17

18 Glued Laminated Timber Service strength [N/mm 2 ], example chart (RIL 205): Strength grade GL28c GL32c GL32h Bending f m,k Tension f t,0,k Tension f t,90,k 16,5 0,4 19,5 0,45 22,5 0,5 Compression f c,0,k 24 26,5 29 Compression f c,90,k 2,7 3,0 3,3 Shear f v,k 2,7 3,2 3,8 f v,90,k = 0,5 f v,k Factor k h, if the height of the beam in bending or longer side in tension (h) < 600 mm: k h = min ( 600 / h) 0, 1 1,1 18

19 Glued Laminated Timber Service stiffness [N/mm 2 ] and density, example chart (RIL 205): : Strength grade GL28c GL32c GL32h Modulus of elasticity: E 0,mean E 0,05 E 90,mean Modulus of shear: G mean Density [kg/m 3 ]: ρ k

20 LVL (Laminated Veneer Lumber) Kertopuu = finnish brand name of LVL KERTO-S beam KERTO-T column KERTO-Q cross-laminated (impregnated) Type approval from Ministry of the Environment LVL: - made of cut and glued veneers - veneer thickness = 3 mm - veneer length >1200mm (Finnish LVL) - scarf joints 20

21 LVL Finnish type approval is required e.g. KERTO-S 111/6621/2000. Service strength and stiffness values correspond to test EC conditions: - relative humidity 65% RH - temperature 20 C - duration 300s (5min) Characteristic values of strength are adjusted to the size of cross section with the factor k h, if the height of the beam in bending or longer side in tension (h) 300 mm, and in tension with the factor k l, if the element length L 3000 mm 21

22 LVL Service strength [N/mm 2 ], example chart (RIL 205): Strength grade KERTO-S KERTO-T KERTO-Q Bending f m,k - on edge/breast 44 / / 32 28/32 32/36 Tension f t,0,k Tension f t,90,k 0,8 0,5 6,0 6,0 Exponent of size effect s 0,12 0,15 0,12 0,12 Compression f c,0,k Compression f c,90,k - on edge/breast 6 / 1,8 4 / 1 9/1,8 9/1,8 Shear f v,k - on edge - on breast/top veneer direction 4,1 2,3 2,4 1,3 4,5 1,3 4,5 1,3 If h 300mm or L 3000mm: k h s 300 = h 1,2 ja k l = 3000 l s / 2 1,1 22

23 LVL stiffness [N/mm 2 ] and density Example chart RIL 205: Strength grade KERTO-S KERTO-T KERTO-Q Modulus of elasticity: E mean E ,05 Modulus of shear: G mean 600 G 0, Density [kg/m 3 ]: ρ 510 mean ρ 480 k

24 Example of size effect The height of cross section changes: factor k h in bending and in tension: Timber GL c/h LVL (K-S/Q) h k h h k h h k h 25 1, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,87 24

25 Service classes Service classes are aimed to assign strength and stiffness values to defined conditions (humidity, temperature) Especially humidity has a great effect on strength and stiffness values of wood Wood is hygroscopic, there is always moisture equilibrium between wood and ambient moisture. E.g. moisture equilibrium of spruce 25

26 Service classes Chart. Moisture of wood ( average percent of moisture in wood, mass of water/mass of wood ) and corresponding relative humidity (RH) of air in 20 C temperature in different service classes. The limits can be exceeded for few weeks in a year. Service class EC5 Relative Humidity (of air) Moisture of wood 1 <65% < 12 % 2 <85% < 20 % 3 >85% >20% 26

27 Description of service classes Service class Descriptions of service classes, examples 1 Heated interiors and structures in insulation layers covering also beams, whose cross section under tension is in insulation layer. 2 Roofed (covered) exteriors, dry wooden structures located outside. 3 Uncovered wooden structures, e.g. structures outside in rain. 27

28 Load-duration classes Wood can bear substantial shortterm loads, but can break under considerably smaller long-term loads. Displacements will also increase in time. 28

29 Load-duration classes Description Permanent: > 10 years Long-term: 6 months - 10 years Middle-term: 1 week - 6 months Short-term: < 1 week Instantaneous Example Self-weight light interior walls soil pressure Stored goods water tanks Snow (surface) live load (classes A-D,F,G) Live load in stairs (point) live load installation loads horizontal loads of interior walls and rails Wind, accidental loads 29

30 Load-duration classes Load-duration class of a load combinations is selected according to the shortest duration in combination. Load combination, examples Self-weight of structures (Sw) Sw + (surface) live load (A-D) Sw + snow Sw + live load in stairs Sw + snow + wind Sw + wind Load-duration class Permanent Middle-term Middle-term Short-term Instantaneous Instantaneous It is important to find the most determining combination in analysis, and sometimes we need to analyse several different combinations to achieve this goal! 30

31 Sources: RIL 205 STEP